Heretofore, it has been difficult to make very thin, i.e., less than 5 mils thick (0.005 inch) films that contain high amounts of filler material (over 25 volume %). As the filled films are calendered, the resultant movement of the filler particles past one another causes the film to tear or form pinholes, i.e. very small holes that cause electrical shorts when used as insulation. In addition, calendering may result in overworking the polytetrafluoroethylene, which tends to cause pinholes.
Highly filled films are desirable because of the wide range of uses for them. The uses will vary depending on how the particles affect the properties of the filled film. For example, a filler with a high dielectric constant can impart dielectric properties to a film that ordinarily would have a low dielectric constant. Reinforcing fillers can impart greater strength to base films. Still other fillers can impart thermal properties to films, and cause the film to act as a thermal sink. Other types of fillers can add electrically conductive properties or shielding properties to films.
Thus, fillers alter the characteristics of plastic films and enable the films to be used in applications they could not otherwise be used in. Very thin films are desirable, also, for applications where space volume or weight is important, as for example, in printed circuit boards where space reduction is desirable, or in electronic shielding applications, or in gaskets where weight and space reduction is important. Unfortunately, the thinner films become, the greater their susceptibility to tearing or forming pin holes due to the shifting and movement of the filler particles within the film caused by the forces on the film generated by the processing equipment.
It would be desirable to provide a process that minimizes the shifting and movement of the particles, and also allows very thin films to be prepared. It would also be desirable to provide thinner, more highly filled films than heretofore possible.